1. Field of the Invention
The present invention relates to a semiconductor substrate, a method for manufacturing the semiconductor substrate, and a method for manufacturing a semiconductor device using the semiconductor substrate.
2. Description of the Related Art
In recent years, as a semiconductor substrate, a silicon on insulator (SOI) substrate in which an insulating layer and a single crystal silicon layer are provided over a supporting substrate (also referred to as a base substrate) has been widely used.
As typical methods for manufacturing an SOI substrate, Smart Cut (registered trademark), epitaxial layer transfer (ELTRAN) (registered trademark), and the like are given.
A typical method for manufacturing an SOI substrate using a glass substrate as a supporting substrate is described below. First, by implanting a hydrogen ion to a single crystal silicon substrate, an embrittled region (also referred to as a damaged region) is formed at a predetermined depth from a surface of the single crystal silicon substrate. Then, the single crystal silicon substrate and the supporting substrate are bonded to each other with an insulating layer interposed therebetween. After that, a heat treatment is performed and the single crystal silicon substrate is cleaved (separated) at the embrittled region, so that the insulating layer and a thin single crystal silicon layer can be formed over the supporting substrate. That is, an SOI substrate is manufactured by transferring the single crystal silicon layer formed of a part of the single crystal silicon substrate to the supporting substrate.
A single crystal silicon layer which is transferred to a supporting substrate by the above-described method for manufacturing an SOI substrate has an uneven surface. Thus, a step for planarizing the surface of the single crystal silicon layer is needed. As a typical example of a method for planarizing the single crystal silicon layer, a method in which a surface of a single crystal silicon layer is irradiated with laser light to melt a depressed portion and a projected portion of the single crystal silicon layer and solidification is performed, so that the single crystal silicon layer is planarized can be given (see Patent Document 1).